The asynchronous transfer mode (ATM) technology is a flexible form of transmission which allows any type of service traffic, voice, video or data, to be multiplexed together on to a common means of transmission. In order for this to be realised, the service traffic must first be adapted typically into 53 byte cells comprising 5 byte headers and 48 byte payloads such that the original traffic can be reconstituted at the far end of an ATM network. This form of adaptation is performed in the ATM adaptation layer (AAL). Five types of adaptation layer have been defined. In particular, adaptation layer 1 is used to adapt constant bit rate traffic to the ATM standard.
A general description of a large capacity ATM cross connect system with an STM-ATM conversion function is given by Kurano et al. in Proceedings of Globecom 93, IEEE Global Communications Conference (ISBN 0-7803-0917-0) Vol.3, pages 1461 to 1467. A discussion of bridge functions used in the implementation of ATM networks is given by Nishara et al. in Proceedings of Globecom 93, IEEE Global Communications Conference (ISBN 0-7803-0917-0) Vol.2, pages 888 to 892. Reference is also directed to our co-pending United Kingdom applications No. 9410294.4, No. 9410295.1 and No. 9411894.0 which relate to integrated broad band and narrow band access arrangements.
ATM switches provide point-multipoint capability as a basic function. A combination of an ATM switch and a narrow band to broad band service interworking capability can therefore achieve point-multipoint in the narrow band network. This solution however has the problem that each narrow band to ATM adaptation incurs a 6 millisecond cell assembly delay which means that voice connections through the existing narrow band network will suffer from delay problems which require the use of additional equipment to effect for cancellation.
The object of the invention is to minimise or to overcome this disadvantage.